This invention relates to a hydraulic control system for controlling the swing of a boom and cab mounted on a swing table of a hydraulically operated crane.
In mobile and stationary cranes it is necessary to swing the cab and boom in order to pick up, move or to set down a load. A swing table may be rotated through a system which is mechanical, such as where a prime mover is connected through a gear system to the swing table, or through a system which is mechanical and hydraulic, such as where a prime mover drives a pump which, in turn, drives a hydraulic motor connected to the swing table. Whenever the swing table is rotating, it is important that the boom always be centered over the load. If the load leads or follows the end of the boom, it is likely that the load will begin to oscillate about the end of the boom resulting in the load hitting something.
When a load is initially lifted and accelerated, the crane operator can normally control the speed of the rotation of the boom sufficiently to prevent the boom from moving out of alignment with the load. A problem arises when it is desired to decelerate the boom and load. If the rotation of the swing table is slowed at too great a rate, the load will begin to lead the boom and begin to oscillate. Consequently, it is desirable to have a system which will provide the minimum amount of drag on the components which are connected to the swing table and will allow the table to swing free and follow the load.
One method for driving a swing table is to have a prime mover drive a torque converter which is geared to the swing table. A problem with using a torque converter to control the rotation of a swing table is the cost of the torque converter. Torque converters are complex transmissions which are extremely expensive.
Another common method of moving a swing table is to have a prime mover drive a fixed displacement pump which, in turn, drives a fixed displacement hydraulic motor having a pinion gear on an output shaft which is meshed with a large bull gear on the crane swing table. Special directional proportional control valves are used to control the flow of fluid from the hydraulic pump to the motor. A problem with using a fixed displacement pump and directional proportional control valves is that a great deal of energy is lost during deceleration of the system.
It is desirable to provide a system for rotating a swing table that is less expensive than a mechanical system incorporating a torque converter and is more efficient than a hydraulic system incorporating fixed displacement pumps and directional proportional control valves.